In the design of integrated circuits it is often necessary to control the quiescent voltage of output signals of a stage, for instance for nullifying it in coincidence with a null input signal. The term “quiescent voltage” of an output signal of a stage indicates the voltage present at the output when the input signal is null. It may represent the DC component of a single-ended signal or the common mode voltage of a differential signal.
At times, it is necessary to insert a buffer stage between two circuits in cascade, for providing on a relatively low impedance node, the signal produced by the upstream circuit to the downstream circuit. In applications as the ones mentioned above, common collector amplifiers are often used, such as a single-ended amplifier depicted in FIG. 1.
A biasing network DC_POLARIZATION fixes the working point of the transistor, that generates an output voltage OUT corresponding to the input signal IN. The quiescent voltage at the output is determined by the polarization of the transistor Qa. When the quiescent voltage of a differential signal, that is its common mode voltage, must be modified, a differential stage as the one shown in FIG. 2 is generally used, which is obtained by connecting two amplifiers of FIG. 1 in parallel, using the same biasing network.
Only for illustration purposes, hereinafter reference will be made to amplifiers realized with bipolar NPN transistors, as shown in the figures, but the same architectures may be realized with PNP transistors or with MOS transistors, as would be appreciated by those skilled in the art. An important requisite of these amplifiers is to have an operation characteristic substantially independent from temperature. Should this not be the case, then the output quiescent voltage would drift and this could cause signal processing errors.
For this reason, the biasing network DC_POLARIZATION usually comprises a band-gap reference voltage generator that is relatively insensible to temperature variations. Therefore, the biasing voltage applied on the base node (or the gate node in MOS technology) is kept substantially constant. Unfortunately, even with this expedient, the quiescent output voltage may still vary with temperature, because the bias current I and the base-emitter (gate-source) voltage inevitably vary with the temperature of operation.